1. Field of the Invention
The present invention relates to an image forming optical system, in particular to an image forming optical system having a high zoom ratio. The present invention also relates to an image pickup apparatus equipped with such an image forming optical system.
2. Description of the Related Art
In recent years, digital cameras and video cameras that pick up an image of an object using a solid state image pickup element such as a CCD or CMOS have replaced film cameras and become the mainstream of the image pickup apparatus. There are various categories of digital cameras ranging from popular-priced compact type cameras to high end function rich cameras.
Users of compact type digital cameras generally wish to enjoy easy shooting in various shooting situations. Therefore, they prefer small size digital cameras with good portability. Size reduction of zoom lenses that are equipped in or attached to such digital cameras are also needed accordingly.
For use with an image pickup element having a large number of pixels, a zoom lens having high optical performance that is matched with the number of pixels is preferred. On the other hand, there are needs for const reduction. In addition, in order to increase the variety of shooting situations, a zoom lens having a wide angle of view at the wide angle end and a high zoom ratio is needed.
Japanese Patent Application Laid-Open Nos. 2008-304708 and 2007-264395 discloses compact zoom lenses having a high zoom ratio that can suitably be used with an image pickup element having a large number of pixels.
Japanese Patent Application Laid-Open No. 2008-304708 discloses a zoom lens including, in order from its object side, a first lens unit having a positive refracting power, a second lens unit having a negative refracting power, a third lens unit having a positive refracting power, a fourth lens unit having a negative refracting power, and a fifth lens unit having a positive refracting power. This zoom lens has a zoom ratio as high as approximately 12.
Japanese Patent Application Laid-Open No. 2007-264395 discloses a zoom lens including, in order from its object side, a first lens unit having a positive refracting power, a second lens unit having a negative refracting power, a third lens unit having a positive refracting power, a fourth lens unit having a negative refracting power, and a fifth lens unit having a positive refracting power. This zoom lens has a zoom ratio as high as approximately 10.